The present invention relates to a method for producing a grain oriented electrical steel strip with good magnetic properties from a continuously cast thin strip. The cast strip is cooled in a manner whereby a grain growth inhibitor needed to develop the grain orientation by the process of secondary grain growth is precipitated as a finely and uniformly dispersed phase. The cast strips produced by the present invention exhibit very good physical characteristics.
Grain oriented electrical steels are characterized by the type of grain growth inhibitors used, the processing steps used and the level of magnetic properties developed. Typically, grain oriented electrical steels are separated into two classifications, conventional (or regular) grain oriented and high permeability grain oriented, based on the level of the magnetic permeability obtained in the finished steel sheet. The magnetic permeability of steel is typically measured at a magnetic field density of 796 A/m and provides a measurement of the quality of the (110)[001] grain orientation, as measured using Millers indices, in the finished grain oriented electrical steel.
Conventional grain oriented electrical steels typically have magnetic permeability measured at 796 A/m of greater than 1700 and below 1880. Regular grain oriented electrical steels typically contain manganese and sulfur (and/or selenium) which combine to form the principal grain growth inhibitor(s) and are processed using one or two cold reduction steps with an annealing step typically used between cold reduction steps. Aluminum is generally less than 0.005% and other elements, such as antimony, copper, boron and nitrogen, may be used to supplement the inhibitor system to provide grain growth inhibition. Conventional grain oriented electrical steels are well known in the art. U.S. Pat. Nos. 5,288,735 and 5,702,539 (both incorporated herein by reference) describe exemplary processes for the production of conventional grain oriented electrical steel whereby one or two steps of cold reduction, respectively, are used.
High permeability grain oriented electrical steels typically have magnetic permeability measured at 796 A/m of greater than 1880 and below 1980. High permeability grain oriented electrical steels typically contain aluminum and nitrogen which combine to form the principal grain growth inhibitor with one or two cold reduction steps with an annealing step typically used prior to the final cold reduction step. In many exemplary processes for the production of high permeability grain oriented electrical steels in the art, other additions are employed to supplement the grain growth inhibition of the aluminum nitride phase. Such exemplary additions include manganese, sulfur and/or selenium, tin, antimony, copper and boron. High permeability grain oriented electrical steels are well known in the art. U.S. Pat. Nos. 3,853,641 and 3,287,183 (both incorporated herein by reference) describe exemplary methods for the production of high permeability grain oriented electrical steel.
Grain oriented electrical steels are typically produced using ingots or continuously cast slabs as the starting material. Using present production methods, grain oriented electrical steels are processed wherein the starting cast slabs or ingots are heated to an elevated temperature, typically in the range of from about 1200xc2x0 C. to about 1400xc2x0 C., and hot rolled to a typical thickness of from about 1.5 mm to about 4.0 mm, which is suitable for further processing. The slab reheating in current methods for the production of grain oriented electrical steels serves to dissolve the grain growth inhibitors which are subsequently precipitated to form a fine dispersed grain growth inhibitor phase. The inhibitor precipitation can be accomplished during or after the step of hot rolling, annealing of the hot rolled strip, and/or annealing of the cold rolled strip. The additional step of breakdown rolling of the slab or ingot prior to heating of the slab or ingot in preparation for hot rolling may be employed to provide a hot rolled strip which has microstructural characteristics better suited to the development of a high quality grain oriented electrical steel after further processing is completed. U.S. Pat. Nos. 3,764,406 and 4,718,951 (both incorporated herein by reference) describe exemplary prior art methods for the breakdown rolling, slab reheating and hot strip rolling used for the production of grain oriented electrical steels.
Typical methods used to process grain oriented electrical steels may include hot band annealing, pickling of the hot rolled or hot rolled and annealed strip, one or more cold rolling steps, a normalizing annealing step between cold rolling steps and a decarburization annealing step between cold rolling steps or after cold rolling to final thickness. The decarburized strip is subsequently coated with an annealing separator coating and subjected to a high temperature final annealing step wherein the (110)[001] grain orientation is developed.
A strip casting process would be advantageous for the production of a grain oriented electrical steel since a number of the conventional processing steps used to produce a strip suitable for further processing can be eliminated. The processing steps which can be eliminated include, but are not limited to, slab or ingot casting, slab or ingot reheating, slab or ingot breakdown rolling, hot roughing and hot strip rolling. Strip casting is known in the art and is described, for example, in the following U.S. Pat. Nos. (all of which are incorporated herein by reference): 6,257,315; 6,237,673; 6,164,366; 6,152,210; 6,129,136; 6,032,722; 5,983,981; 5,924,476; 5,871,039; 5,816,311; 5,810,070; 5,720,335; 5,477,911; and 5,049,204. When employing a strip casting process, at least one casting roll and, preferably, a pair of counter rotating casting rolls is used to produce a strip that is less than about 10 mm in thickness, preferably less than about 5 mm in thickness and, more preferably, about 3 mm in thickness. The application of strip casting to the production of grain oriented electrical steels differs from processes established for the production of stainless steels and carbon steels due to the technically complex roles of the grain growth inhibitor system (such as MnS, MnSe, AIN and the like), grain structure and crystallographic texture which are essential to produce the desired (110)[001] texture by secondary grain growth.
The present invention relates to a process for producing grain oriented electrical steel from a cast strip wherein rapid secondary cooling of the cast strip is employed to control the precipitation of the grain growth inhibiting phases. The cooling process can be accomplished by the direct application of cooling sprays, directed cooling air/water mist, or impingement cooling of the cast strip onto solid media such as a metal belt or sheet. While the cast strip is typically produced using a twin roll strip caster, alternative methods using a single casting roll or a cooled casting belt may also be used to produce a cast strip having a thickness of about 10 mm or less.
Specifically, the present invention provides a method for producing grain oriented electrical steel strip comprising the steps of:
(a) forming a continuously cast electrical steel strip having a thickness of no greater than about 10 mm;
(b) cooling said strip to a temperature of from about 1150xc2x0 C. to about 1250xc2x0 C. such that it becomes solidified; and
(c) subsequently performing a rapid secondary cooling on said steel strip wherein the strip is cooled at a rate of from about 65xc2x0 C./second to about 150xc2x0 C./second to a temperature of no greater than about 950xc2x0 C.
In one embodiment, the steel strip produced by the foregoing process is coiled at a temperature below about 850xc2x0 C., preferably below about 800xc2x0 C.
In another embodiment, the present invention provides a method for producing a grain oriented electrical steel strip comprising the steps of:
(a) forming a continuously cast electrical steel strip having a thickness of no greater than about 10 mm;
(b) cooling said strip to a temperature below about 1400xc2x0 C. such that it becomes at least partially solidified;
(c) performing an initial secondary cooling on said solidified strip to a temperature of from about 1150xc2x0 C. to about 1250xc2x0 C.; and
(d) subsequently performing a rapid secondary cooling on said steel strip wherein the strip is cooled at a rate of from about 65xc2x0 C./second to about 150xc2x0 C./second to a temperature of no greater than about 950xc2x0 C.
In one embodiment of this invention, the steel strip produced by the foregoing process is coiled at a temperature below about 850xc2x0 C., preferably below about 800xc2x0 C.
This process provides a grain oriented electrical steel having the appropriate grain orientation, and also provides steel with good physical properties, such as reduced cracking.
For purposes of clarity, the rate of cooling during solidification will be considered to be the rate at which the molten metal is cooled through the casting roll or rolls wherein the substantially solidified cast strip is cooled to a temperature at or above about 1350xc2x0 C. The secondary cooling of the cast strip will be considered divided into two stages: (i) initial secondary cooling is conducted after solidification to a temperature range of about 1150 to 1250xc2x0 C., and, (ii) rapid secondary cooling is employed after the strip is discharged from the initial cooling and serves to control the precipitation of the grain growth inhibiting phase(s) present in the steel.
Prior to initiation of rapid secondary cooling, it is an optional feature of the present invention to slow the rate of initial secondary cooling of the cast strip to allow the strip temperature to equalize before initiating rapid secondary cooling. For example, the cast and solidified strip may be discharged into and/or pass through an insulated chamber (see FIG. 1) to both slow the initial secondary cooling rate and/or to equalize the strip temperature after solidification. Although not critical to the practice of the present invention, a nonoxidizing atmosphere may be optionally used in the chamber to minimize the surface scaling, thereby helping to maintain a low surface emissivity which can further slow the rate of initial secondary cooling preceding the rapid secondary cooling of the present invention. These optional configurations are helpful as they permit rapid secondary cooling of the solidified strip to be conducted at a substantially greater distance from the strip casting machine, thereby, providing some isolation of the liquid steel handling and strip casting equipment from the rapid secondary cooling equipment. In this manner, any negative interaction between the media used for the rapid secondary cooling process of the present invention and the liquid steel handling and/or strip casting process and/or equipment can be minimized. For example, if a water spray or a water/air mist is used as the cooling media, the liquid steel and/or strip casting equipment must be protected from any steam formed as a result of rapid secondary cooling. Moreover, conducting both the initial and rapid secondary cooling in a nonoxidizing atmosphere will minimize metal yield losses due to oxidation of the strip during cooling.
During solidification, the liquid metal is cooled at a rate of at least about 100xc2x0 C./second to provide a cast and solidified strip having a temperature in excess of about 1300xc2x0 C. The cast strip is subsequently cooled to a temperature of about 1150xc2x0 C. to about 1250xc2x0 C. at a rate of at least about 10xc2x0 C./second, whereupon the strip is subjected to rapid secondary cooling to reduce the strip temperature from about 1250xc2x0 C. to about 850xc2x0 C. In the broad practice of this invention, rapid secondary cooling is conducted at a rate of at least about 65xc2x0 C./second while a preferred cooling rate is at least about 75xc2x0 C./second, and a more preferred rate is at least about 100xc2x0 C./second. The cast and cooled strip may be coiled at a temperature below about 800xc2x0 C. for further processing.
In the practice of the invention, several methods for the rapid secondary cooling have been employed such as direct impingement cooling to provide a cooling rate at or in excess of about 150xc2x0 C./second or water spray cooling to provide a cooling rate at or in excess of about 75xc2x0 C./second. It has been further found in the development of the present invention that producing a cast and rapidly cooled electrical steel strip with good mechanical and physical characteristics may limit the rate of rapid secondary cooling. Rapid secondary cooling at rates in excess of about 100xc2x0 C./second requires that the strip be cooled in a manner which prevents significant temperature differentials to develop during cooling since the strain created by differential cooling has been found to result in cracking of the cast strip, making the cast strip unusable for further processing.
The conditions for the rapid secondary cooling steel strip may be controlled using a system comprising a spray nozzle design wherein the rapid cooling is provided by establishing a desired spray water density. The spray density may be controlled by the water flow rate, the number of spray nozzles, the nozzle configuration and type, spray angle and length of cooling zone. It has been found that a water spray density of from about 125 liters per minute per square meter of surface area (l/[min-m2]) to about 450 l/[min-m2] provides the desired cooling rate. Since it is difficult to monitor the strip temperature during water spray cooling due to the variations in and turbulence of the water film applied onto the strip, water spray density measurements are typically used.
The term xe2x80x9cstripxe2x80x9d is used in this description to describe the electrical steel material. There are no limitations on the width of the cast material except as limited by the width of the casting surface of the roll(s). The cast and cooled strip is typically further processed using hot and/or cold rolling of the strip, annealing of the strip prior to cold rolling to final thickness in one or more stages, annealing between cold rolled stages if more than more than one cold reduction stage is used, decarburization annealing of the finally cold rolled strip to lower the carbon content to less than about 0.003%, applying an annealing separator coating such as magnesia, and a final annealing step wherein the (110)[001] grain orientation is developed by the process of secondary grain growth and the final magnetic properties are established.